The present invention relates to a semiconductor epitaxial wafer for light emitting diodes or LEDs for short, and its fabrication method.
In recent years compound semiconductors have often been used as optical semiconductor device materials. In semiconductor materials used to this end, epitaxially grown layers of desired semiconductor crystals are formed on single crystal substrates. This is because currently available crystals, especially those usable as substrates have many defects and low purity as well, and so much difficulty is involved in using them immediately for light emitting devices. For this reason, a layer having the composition to obtain the emission of light of desired wavelength is epitaxially grown on a substrate. A ternary crystal layer is used primarily for this epitaxially grown layer. For such epitaxial growth, usually, vapor- and liquid-phase growth processes are employed. In the vapor-phase growth process, a holder made of graphite or quartz is mounted within a reactor made of quartz, in which raw feed gases are continuously fed and heated for epitaxial growth.
Semiconductors of the groups III-V compounds have a bandgap corresponding to the wavelengths of visible light and infrared light, and so are being applied to light-emitting devices. Of these, GaAsP and GaP are widely used as LED materials in particular.
Referring here to GaAs.sub.1-x P.sub.x as an example, GaAs.sub.1-x P.sub.x where 0.45&lt;.times..ltoreq.1 is doped with nitrogen (N) as an isoelectronic trap for trapping conduction electrons, so that a light-emitting diode can be obtained, which increases about tenfold in terms of light output. Thus, GaAs.sub.1-x P.sub.x where 0.45 &lt;.times..ltoreq.1.0 grown on a GaP substrate is usually doped with nitrogen.
FIG. 2 illustrates one construction of a conventional GaAsP epitaxial wafer.
In the vapor-phase growth process, raw feed gases are continuously fed in a reactor for growing an N-type epitaxial layer on an N-type GaP substrate 1. To prevent the occurrence of lattice strains due to a difference in lattice constant between the substrate and the epitaxial layer, a GaAsP-type layer 2 having a stepwise or continuous varying composition is formed as an intermediate layer to form GaAsP-type layers 3 and 4, each having a constant composition, and the layer 4 is doped with nitrogen, or N, in the form of an isoelectronic trap. At a post-doping step, zinc is thermally diffused at high concentration in the layer 4 to form a P-type layer 5 of about 4 to 10 .mu.m on the surface of the epitaxial layer. Thus, the formed P-type layer has a relatively constant carrier concentration, and good-enough ohmic contact is achievable in a stable manner. The thermal diffusion process enables about dozens of epitaxial wafers to about one hundred to be processed at one time, and offers a cost advantage. Generally, therefore, P-type layers are formed by the thermal diffusion process after N-type layers alone have been grown by the vapor-phase growth process.
By doing this it may be possible to obtain LEDs in a stable manner. However, the carrier concentration of the P-type layer 5 above the PN junction region becomes too high with an increased absorption of light, resulting in a drop of LED light output. In addition, the PN junction region is thermally damaged to incur deterioration of the crystals forming the epitaxial layer. Although these problems may be solved by decreasing the diffusion temperature extremely, yet the P-type layer becomes too thin to obtain good-enough ohmic contact due to a lowering of the surface carrier concentration.
As mentioned above, the vapor-phase grown epitaxial wafer has both the epitaxial layer and the GaP substrate of the N-conduction type. In the epitaxial growth of GaAsP, generally, zinc may be used as a dopant to grow a P-type layer during vapor-phase growth. If doping is carried out with vapor-phase growth using a P-type dopant zinc in the form of diethylzinc gas, the possible highest concentration is about 5.times.10.sup.18 cm.sup.-3 due to a high growth temperature when the most prevalent hydride transport process is used; in other words, it is difficult to carry out doping at a concentration higher than that. Nonetheless, since the carrier concentration of the P-type layer in the vicinity of the PN junction is lower than that, the absorption of light by the P-type layer is reduced and the PN junction is formed by vapor-phase growth at one time, so that good-enough crystallinity can be imparted thereto. In addition, the thus obtained P-type layer accomplishes an about 20 to 30% improvement in terms of light output as compared with that formed by thermal diffusion. It is here to be noted that even though growth conditions are adjusted to achieve a high concentration of zinc doping for vapor-phase growth, the crystallinity of the epitaxial layer will become worse.
To form an electrode of good-enough ohmic contact on a P-type layer with semiconductors of the groups III-V compounds, it is usually required that the carrier concentration of the P-type layer be at least 1.times.10.sup.19 cm.sup.-3. To obtain good-enough ohmic contact, various procedures have heretofore been used; for instance, various combinations of AuZn, AuNiZn and other materials are selected as electrode materials, or electrodes of multilayered structures or varying compositions are used. Still, the most effective procedure is to increase the carrier concentration of the P-type layer. For example, GaAs has been doped with zinc at a concentration higher than 5.times.10.sup.18 cm.sup.-3. When the doping amount of zinc, i.e., the carrier concentration is increased by elevating the diffusion temperature of zinc, however, the P-type layer rather absorbs the emitted light, and crystal defects due to thermal strains or the like occur, resulting in a drop of LED's light-emission output. The light output of an LED may be improved by decreasing the concentration of zinc to about 5.times.10.sup.18 cm.sup.-3 or less by lowering the diffusion temperature of zinc. However, the carrier concentration drops, thus making it difficult to form an ohmic electrode on the P-type layer and, hence, giving rise to a forward voltage variation or increase.
According to the present invention accomplished with such situations in mind, it has now been found that in an epitaxial wafer including a PN junction there is a difference in the optimum carrier concentration between a PN junction region and an ohmic contact region in a P-type layer. Thus, an object of the present invention is to provide a P-type layer of the optimum structure that makes it possible to achieve an improvement in light output and form a good-enough ohmic electrode, and a fabrication method effective for such an epitaxial wafer as well.
As a result of intensive studies made for the purpose of solving such a problem, it has now been found that a P-type layer should comprise two layers and that the carrier concentration that is the greatest factor in determining the characteristics of an LED should be optimized, whereby good-enough ohmic contact can be achieved in a stable manner and the light output of an LED can be 20 to 30% higher than achieved so far in the art as well.